Some More MAC Features
Use of RTS/CTS is controlled by an RTS threshold.
RTS/CTS is only used for data packets longer than the RTS threshold.
Pointless to use RTS/CTS for short data packets – high overhead!
Number of retries is limited by a Retry Counter.
Short retry counter: for packets shorter than RTS threshold.
Long retry counter: for packets longer than RTS threshold.
Packets can be fragmented.
Each fragment is acknowledged.
But all fragments are sent in one sequence.
Sending shorter frames can reduce impact of bit errors.
Lifetime timer: maximum time for all fragments of frame.
Fragmentation
Power management (infrastructure)
APs buffer packets to stations in power saving mode.
APs announce in beacons which packets are waiting with the TIM (traffic indication Map).
Broadcast/multicast frames are also buffered at AP.
Sent after beacons, same common timing period.
Uses Delivery Traffic Indication Map (DTIM).
AP controls DTIM interval.
STA in power save wake periodically to listen for beacons.
If it has data pending, send a PS-Poll.
AP sends buffered data to this PS-poll.
TSF (Timing Synchronization Function) assures AP and stations are synchronized.
Synchronizes clocks of the nodes in the BSS.
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